Photocatalytic degradation of humic acid in the presence of montmorillonite

Humic acids (HA) representing the major fraction of the naturally occurring humic substances (HS) are composed of highly functionalized carbon rich polydisperse organic fractions. Clay minerals which are responsible for the transport of inorganic and organic contaminants constitute the main component of the dispersed inorganic material in natural waters. The understanding of interactions between humic substances and clay minerals is an important task for the achievement of an effective water treatment performance. The aim of this research was to investigate the influence of montmorillonite as a representative clay mineral on the TiO2 photocatalytic removal of humic acids as the model compound of natural organic matter. The interactions prevailing between humic acid, montmorillonite and TiO2 surface were assessed prior to the application of photocatalysis in order to address the adsorptive and photolytic behavior of humic acids. The changes attained in humic acid were described by UV–vis spectroscopic i.e. color forming moieties (Color436) and UV absorbing centers (UV365 and UV254), and dissolved organic carbon (DOC) contents. Application of the pseudo first order kinetic model revealed both an enhancement and retardation with respect to the applied montmorillonite dose. The overall effect of montmorillonite on the photocatalytic degradation of humic acid was also evaluated in terms of molecular size distribution profiles (0.45 μm filtered fraction, 100 kDa fraction, 30 kDa fraction and 3 kDa fraction) described by the specified and DOC normalized specific UV–vis parameters.